The present invention relates to a method of fabricating a composite material part that comprises fiber reinforcement densified by a matrix.
The field of application of the invention relates more particularly to thermostructural composite materials, i.e. composite materials having good mechanical properties and the ability to conserve these properties at high temperature. Typical thermostructural composite materials are carbon/carbon (C/C) composite materials made up of carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) materials made up of refractory fiber reinforcement (carbon fibers or ceramic fibers) densified by a matrix that is made of ceramic, at least in part. Examples of CMCs are C/SiC composites (carbon fiber reinforcement and silicon carbide matrix), C/C—SiC composites (carbon fiber reinforcement and a matrix comprising both a carbon phase, generally close to the fibers, and a silicon carbide phase), and SiC/SiC composites (reinforcing fibers and matrix made of silicon carbide). An interphase layer may be interposed between reinforcing fibers and the matrix in order to improve the mechanical behavior of the material.
Fabricating a thermostructural composite material part generally comprises making a fiber preform of a shape that is close to the shape of the part that is to be fabricated, and then densifying the preform with the matrix.
The fiber preform constitutes the reinforcement of the part and its function is essential for obtaining good mechanical properties. The preform is obtained from fiber textures: yarns, tows, braids, woven fabrics, felts, . . . . Shaping is generally performed by winding, weaving, stacking, and possibly needling together two-dimensional plies of woven fabric or sheets of tows, . . . .
Densifying the fiber preform consists in filling the pores in the preform with the material constituting the matrix, which material occupies all or a fraction of the volume of the preform.
The matrix of a thermostructural composite material may be obtained using various known techniques, and in particular using a liquid or a gas.
The liquid technique consists in impregnating the preform with a liquid composition containing an organic precursor of the matrix material. The organic precursor is usually in the form of a polymer, such as a resin, and it is optionally diluted in a solvent. The precursor is transformed into a refractory phase by heat treatment, after eliminating the optional solvent and curing the polymer. The heat treatment consists in pyrolyzing the organic precursor in order to transform the organic matrix into a carbon or ceramic matrix depending on the precursor used and on the conditions of pyrolysis. By way of example, liquid precursors for carbon may be resins having a relatively high coke content, such as phenolic resins, whereas liquid precursors for ceramic, in particular for SiC, may be resins of the polycarbosilane (PCS) or of the polytitanocarbosilane (PTCS) or of the polysilazane (PSZ) type. A plurality of consecutive cycles going from impregnation to heat treatment may be performed in order to achieve the desired degree of densification.
The gas technique consists in chemical vapor infiltration. The fiber preform is placed in an oven into which a reaction gas is admitted. The pressure and the temperature that exist within the oven, and the composition of the gas, are all selected in such a manner as to enable the gas to diffuse within the pores of the preform in order to form the matrix therein by depositing a solid material in contact with the fibers, which material results from components of the gas decomposing or from a reaction between a plurality of its components. By way of example, gaseous precursors of carbon may be hydrocarbons that produce carbon by cracking, such as methane, and a gaseous precursor of a ceramic, in particular of SiC, may be methyltricholorosilane (MTS) that gives SiC by decomposition of the MTS (optionally in the presence of hydrogen).
Except when the texture already presents the desired shape and fiber fraction, preparing a thermostructural composite material by densification using a gas technique generally begins with a so-called “consolidation” stage that serves to freeze both the shape of the fiber preform and also the fiber fraction of the material (i.e. the percentage of the total apparent volume of the material that is actually occupied by the fibers).
Using a gas technique for consolidation presents a certain number of drawbacks. In order to keep the preform in the desired shape during chemical vapor infiltration for the consolidation step, it is necessary to hold the preform by using tooling made of graphite. Graphite tooling is complex and expensive to make. In addition, graphite tooling ages quickly since it densifies at the same time as the preforms it is holding in shape. Finally, such tooling occupies a significant fraction of the working volume within the CVI oven and it presents a large amount of thermal inertia.
For these reasons, the fiber preform is preferably consolidated using a liquid technique. The fiber texture constituting the preform is impregnated with an organic precursor for the matrix, and it is then shaped by means of tooling (mold or shaper) made of metal or composite material that is reusable and that presents an implementation cost that is much smaller than the cost of tooling made of graphite. Thereafter, the precursor, possibly after drying and polymerizing, is transformed by heat treatment so that, after pyrolysis, there remains a solid residue that serves to consolidate the preform, thus enabling the preform to be placed on its own (i.e. without being held by tooling) in a CVI oven so as to continue densification using a gas technique.
Nevertheless, the thermomechanical characteristics of thermostructural composite materials that have been consolidated and densified by a gas technique are much better than those of known composite materials that have first been consolidated by a liquid technique and then densified by a gas technique. This difference in performance can be explained in particular by the fact that during consolidation by the liquid technique, the matrix that is obtained by the liquid technique is concentrated within the yarns of the composite, thereby preventing the yarns themselves being densified by the matrix obtained when using the gas technique during the subsequent densification step. In addition, consolidation using a liquid technique always leaves residual pores within the yarns because of the inevitably incomplete nature of the transformation of the liquid precursor, given that the pyrolytic yield of organic precursors is always less than 100%. These pores are generally difficult to access from the surfaces of the yarns and consequently they cannot be filled during the subsequent densification by the gas technique. Unfortunately, in order to obtain good mechanical characteristics, it is important to minimize as much as possible any heterogeneous porosity within the composite material.
Furthermore, since the thermomechanical characteristics of the matrix obtained by the liquid technique are generally less good than those of a matrix deposited by a gas technique, composite materials in which the yarns contain in the great majority only the matrix obtained by the liquid technique present thermomechanical properties that are less good.
When using a carbon-precursor resin (e.g. phenolic resin) for consolidating CMC materials, because the coke obtained after the resin has been pyrolyzed presents very fast reaction kinetics in oxidation, the strength of the material for long duration utilization in an oxidizing atmosphere is degraded whenever the temperature exceeds 400° C.
Several options exist for improving the thermomechanical properties of CMC materials that have been consolidated by a liquid technique using a carbon-precursor resin. A first option consists in reducing the coke content present in the resin. A second option consists in using fillers that enable a glass to be formed that provides protection against oxidation.
Nevertheless, the extent to which the quantity of consolidation resin can be reduced in order to reduce the coke content is limited, since below a certain quantity of coke the consolidating effect is no longer ensured. Furthermore, using healing fillers, i.e. fillers that enable a glass to be formed to provide protection against oxidation, becomes effective only from the temperature at which protective glass is formed by oxidizing the fillers. The fillers generally used are fillers containing boron (B4C, SiB6, TiB2, etc.), since they make it possible to obtain boron oxide (B2O3), which is a protective glass have a low softening point. Nevertheless, such fillers can produce a protective effect only at temperatures above 500° C.
To summarize, although the liquid technique greatly simplifies consolidating a fiber texture in comparison with the gas technique, it nevertheless prevents the matrix obtained by the gas technique from being deposited all the way to the cores of the yarns during subsequent densification. In addition, when using a carbon-precursor resin, pyrolysis of the resin leaves residual coke within the yarns, thereby reducing the mechanical strength of the material at high temperatures, and degrading its ability to withstand oxidation.
Consequently, there exists a need to obtain composite materials that present improved thermomechanical characteristics, while conserving the advantages of consolidation by a liquid technique.